1. Field of the Invention
The invention relates to starch phosphates with enhanced properties in regard to application techniques; to a method for the production of such starch phosphates; as well as to their use. Starch phosphates are starch derivatives in which at least some of the available hydroxy-groups of the polysaccharide are esterified with phosphate ester groups with or without cross-linking of the polysaccharide. Non-cross-linked monoesters and cross-linked diesters of the starch are obtained depending on the phosphorylating agents as employed and the conditions of the reaction.
2. The Prior Art
Only starch phosphates with low degrees of substitution have gained technical significance until now.
GB-A 968 427 describes the reaction of corn starch with 0.135 mol NaH2PO and 0.27 mol urea each per equivalent of anhydroglucose, at a vacuum of 77.860 kPa and a temperature of up to 138xc2x0 C. over a time duration of 3 hours. In said process, a water-containing suspension with 45% starch is used as the starting material, which, after adding the other two reaction components, is filtrated prior to the thermal treatment, whereby substantial amounts of urea and phosphate are jointly separated. For that reason, only a starch phosphate with low degrees of substitution of the carbamide and phosphate ester groups is obtained, which in each case amounts to below about 0.07.
In the literature, WALY ET AL, Starch 46 [1994], No. 2, pages 59 to 63, describes among other things the reaction of 50 g starch with 5 g H3PO4 and 2 g urea at a temperature of 155xc2x0 C. over a time duration of 2 hours. Because of the low amount of urea used, the latter only serves as a condensing agent in the reaction, so that no formation of carbamide groups can occur, or occur only to a minor degree.
ES-A-8,401,769 describes a reaction of 65 g starch with 25 g urea and 10 g phosphoric acid, at 150xc2x0 C. over a time of 45 minutes. A starch phosphate is formed as a result of the relatively high proportions of urea and phosphoric acid, because the reaction is carried out at a high temperature and without a vacuum. The starch phosphates so obtained are insoluble products with little swelling power.
It is also from the literature (Houben-Weyl [1987], volume 20/3, page 2164) that the esterification reaction for forming the starch phosphates can be favorably influenced by adding organic nitrogen compounds such as, for example urea. In the specified example, the amount of urea used comes to only 4% based on the starting amount of potato starch. The mixture, which is comprised of the starch, the phosphorylating agent and the urea, is heated for 3 hours to 95xc2x0 C. in order to remove the water, and subsequently tempered for 3.5 hours at 150xc2x0 C., in each case at a reduced pressure of below 7.5 kPa. Based on the contents of phosphorus of 0.31% by weight and nitrogen of 0.08% by weight, the degree of substitution for the phosphate ester groups achieved on the average amounts to below 0.02. The low amount of urea used exclusively serves the purpose of supporting the condensation reaction. The starch phosphate obtained following cooling is soluble in water within just a few seconds.
Furthermore, it is known also (Starch 43 [1991], No. 2, pages 66 to 69) that highly phosphorylated starches with degrees of substitution of about 1 are afflicted with the general drawback that they readily hydrolyze in humid air. The pastes of said starch, moreover, exhibit a lower viscosity than the starting starch.
Furthermore, their preparation requires the use of particularly aggressive phosphorylating agents such as phosphoroxy chloride, phosphorous pentoxide and organic, solvents, whereby the risk of massive formation of di- and tri-starch phosphate groups is posed (Houben-Weyl [1987], vol. 2/3, page 2164). With less aggressive phosphorylating agents such as, for example sodium phosphate, on the other hand, only low degrees of substitution of the starch are achieved.
The invention is based on the problem of providing starch phosphates that have a high degree of substitution; which are resistant to hydrolysis; and which can be produced by means of phosphorylating agents that have only low toxicity. Furthermore, the problem is to provide a suitable method for producing such starch phosphates.
Said problem is solved according to the invention by means of the features specified in the patent claim 18. The features required for carrying out the method are specified in claim 23. Suitable variations of the starch phosphates as well as of their production are specified in claims 19 and 22 and 24 to 29. Special applications of the starch phosphates as defined by the invention are the objects of claims 30 to 33.
By means of combined phosphorylation and carbamidation of the starting starch and the use of nontoxic phosphorylating agents, which are otherwise only employed for producing starch phosphates with low degrees of substitution, it has been possible to obtain highly substituted starch phosphates that are resistant to hydrolysis under normal conditions and exhibit excellent properties in terms of application technology. By means of varying the amounts of phosphoryling agents as well as of the urea used, and/or the reaction times, it is possible to produce starch phosphates with different degrees of substitution of greater than 0.01, preferably ranging from 0.1 to 1 both for the carbamide groups and the phosphate ester groups. Both pure amylopectin and also pure amylose as well as their native mixtures are suitable as starch or starch-containing materials. Mixtures with high starch contents such as flours of the various types of grain, and also pea and potato starches can be used as well. Furthermore, as starches with a slightly reduced molecular weight, so-called thin-dissolving starches as they are obtained by a careful acid treatment, are suitable as well; however, not suitable are dextrins that are produced by a high-degree of acid degradation of starches. Also the so-called soluble starches, i.e. starches soluble in cold water, which are produced by a thermal treatment of starches, can be employed as starting substances.
The urea to be used is not required to satisfy any special requirements; any technical urea quality is suitable. As phosphorylating agent, it is possible to use phosphoric acid of any desired technical quality, whereby the commercially available 85%-quality is particularly suited. When using more highly diluted acids, only more water has to be evaporated prior to the reaction. Instead of using phosphoric acid, it is possible also to employ ammonium phosphates, potassium phosphates or sodium phosphates, as well as mixtures of said phosphates. The use of said phosphates requires by 10 to 15xc2x0 C. higher reaction temperatures as compared to phosphoric acid, but leads to the benefit that fewer starch degradation products are formed due to the lower content of H+-ions. The reaction takes place under reduced pressure of below 13.3 kPa. The process of the carbamidation and phosphorylation reactions under vacuum takes place in a particularly careful manner, and at temperatures that are clearly lower than under normal pressure.
Based on the desired properties of the starch phosphates in terms of application technology, the degrees of, substitution of the carbamide groups and phosphate groups are within a range of greater than 0.01. The degrees of substitution are preferably in a range of from 0.1 to 1 because starch phosphates with a DS of the carbamide groups (DSC) of less than 0.1 have no shelf stability, and starch phosphates with a DS of the phosphate ester groups (DSP) of less than 0.1 are only inadequately expandable or soluble in cold water. Starch phosphates with a DSC or DSP of greater than 1 are poorly biodegradable.
Another substantial advantage offered by the procedure as defined by the invention consists in that starch phosphates with different properties are obtained through reactions carried out at different temperatures. Reaction temperatures of up to 110xc2x0 C. supply starch phosphates that can be processed to homogeneous solutions. At reaction temperatures in excess of 120xc2x0 C., starch phosphates are obtained that can be processed to suspensions of swollen particles with high water retention power, their properties being similar to those of the known super-absorbers.
The starch phosphates produced according to the procedure as defined by the invention are completely incombustible and biodegradable. The relatively high degrees of substitution of the phosphate ester and carbamide groups do not seem to have any adverse effect on the microbial degradation. The starch phosphates, which are homogeneously soluble in water, are particularly suited for use as components for adhesives and for regulating the viscosity of aqueous systems. The high concentration of ionogenic groups has a highly advantageous effect on the dispersion of solid particles.
The starch phosphates, which swell only in water, can be employed like the known super-absorbers as thickening agents, as additions to water for extinguishing fire, and because of their high absorptive capacity as an addition to or as the only component especially of hygienic articles.
Owing to the good biodegradability of the starch phosphates as defined by the invention, the products manufactured from such starch phosphates exhibit substantial superior qualities as compared to polyacrylate products.
Starch phosphates forming gels exhibit the property of exchanging cations and, to the extent they are dissolved homogeneously, are capable of forming complexes. Polyvalent metal ions, in particular heavy metal ions, but also agents supplying hardness are bonded in the process in a fixed manner. The gel-forming products are therefore particularly suited for bonding solutions containing heavy metals because the heavy metals remain fixed in the gel and are not washed out, for example by rain or the like. Gels and clear-dissolving products can be employed for softening water, for example as additives for detergents. The amounts of phosphate added in this way to the waste water are substantially lower than when using polyphosphates as water softeners.